Tour the recent Networld+Interop floor, and one thing was evident:
Gigabit Ethernet has truly arrived. A little more than a year after
products began shipping; and six months after the standard for optical
fiber was ratified, Gigabit Ethernet has become a mature technology
ready to take on a variety of enterprise roles.

Initially, Gigabit Ethernet is finding use mainly as an upgrade for
FDDI (Fiber Distributed Data Interface) find Fast Ethernet backbones.
However, it's expected to quickly follow the same migration path
taken by Fast Ethernet, starting with switch-to-switch links before
moving down the network hierarchy to switch-to-server connections. Once
the standard for unshielded twisted-pair wiring is ratified (see
sidebar), Gigabit Ethernet may even be deployed to the desktop.

Dataquest, the San Jose, Calif., market research firm, expects
Gigabit Ethernet sales to soar from $19.8 million last year to about
$1.36 billion by 2001, with the number of ports shipped rising from
8,100 last year to 1.55 million in 2001.

The Dell' Oro Group of Portola Valley, Calif., a research firm
that tracks the Gigabit Ethernet market, predicts even stronger growth,
based on early indications that users are embracing the technology more
aggressively than expected. Dell'Oro expects shipments to reach 7.8
million ports annually by 2001 and is projecting sales in excess of $3
billion by 2002.

More than 40 vendors now supply Gigabit Ethernet products, ranging
from Layer 2 and Layer 3 (routing) switches, uplink/downlink modules,
and network interface cards (NICs) to router interfaces and a new device
called a full-duplex repeater or buffered distributor. This hub-like
device interconnects two or more Ethernet links operating at 1 Gbps or
faster and forwards all incoming packets to the connected links, except
the originating one, providing a shared-bandwidth domain. Unlike a
repeater, it can buffer incoming frames before forwarding them. It is
useful for connecting server farms to a Gigabit Ethernet backbone.

BACKBONE OPTIONS

About 70 percent of today's network backbones use FDDI, which
is limited to 100 Mbps. This may have been adequate before organizations
began using the Internet and corporate intranets, when 80 percent of
traffic was contained within workgroups and departments. Today, 80
percent of the traffic is on the backbone, pressuring network managers
to find faster alternatives.

Shared FDDI backbones can be upgraded to Gigabit Ethernet by
replacing the backbone FDDI concentrator with a Gigabit Ethernet switch
and installing Gigabit Ethernet NICs in the routers attached to the
backbone. If the network includes a ring of FDDI concentrators or
routers, as in a campus configuration, a Gigabit Ethernet switch or
repeater can be installed as the core of the network, with the routers
attached to the switch.

Similarly, a Fast Ethernet backbone switch that aggregates multiple
10- and 100-Mbps switches can be easily upgraded with a gigabit version.

As a backbone technology, Gigabit Ethernet is expected to face
stiff competition from Asynchronous Transfer Mode (ATM) technology,
especially where voice or video accounts for a large portion of the
traffic. Gigabit Ethernet proponents argue that it is faster and cheaper
than ATM and is easier to implement and manage, since the underlying
technology is widely deployed and well understood.

Gigabit Ethernet also integrates smoothly into existing Ethernet
networks, since it uses the same frame structure and protocol. In
addition, most Gigabit Ethernet switches include 10- and 100-Mbps
Ethernet ports and can operate with the same network management systems.

Even so, ATM is a more mature technology, with established
quality-of-service (QoS) standards, and it integrates well with
Token-Ring networks and the ATM-based carrier networks used for WAN
access. The IEEE 802 committee has established QoS standards for
Ethernet, but these have yet to be universally embraced by Gigabit
Ethernet vendors. Implementing QoS over a WAN link can also be
difficult.

Where QoS is important--say for giving priority to video streams or
determining which traffic will get the most bandwidth from relatively
slow WAN links--ATM may have the edge. Otherwise, Gigabit Ethernet
generally wins on the basis of speed, cost, ease of installation and
configuration, and staff familiarity with the technology.

HIERARCHY UPGRADE

Gigabit Ethernet also provides a simple upgrade path for Fast
Ethernet switches and links, since it uses the same frame formats and
flow control methods, eliminating the need to translate between
different types of Ethernet traffic. In addition, the same network
management systems and troubleshooting techniques can often be used.

As a rule of thumb, wherever Fast Ethernet runs today, Gigabit
Ethernet could well run tomorrow. According to a recent study by the
Cahners In-Stat research firm, 34 percent of enterprises currently use
Fast Ethernet to link switches, while 46 percent use it to link servers.
Much of Gigabit Ethernet's growth will come from upgrading these
links to 1 Gbps.

Before using Gigabit Ethernet to link servers, though, network
managers need to make sure the servers can handle its blazing speed.
Many high-end Unix servers are up to the challenge, but Windows NT 4.0
is questionnable, and users may have to wait for NT 5.0 before
proceeding with the 1-Gbps connection.

IBM is addressing the issue by using larger-than-standard Ethernet
frames, which are easier for servers to handle on high-speed networks.
Developed by Alteon Networks, the so-called jumbo frames contain 9,018
bytes, compared to the Ethernet limit of 1,518 bytes. They improve
performance by requiring the server CPU to handle fewer frames and deal
with fewer interruptions.

IBM will initially support jumbo frames on its RS/6000 servers, and
is expected to add support for its AS/400 mid-range systems and
Netfinity PC server. While jumbo frames is not a standard, it may be
soon if Alteon can make its case with the IEEE committee. IBM's
endorsement may help.

RELATED ARTICLE: Standards continue torrid pace

As with the other Ethernet standards, the ones or Gigabit Ethernet
appear to be on a fast track. Fast Ethernet took only 13 months to go
from first draft to final approval, and Gigabit Ethernet required about
the same time. Last June, the 802.3z task force created by the IEEE
standards committee to develop the 1-Gbps specifications was successful
in ratifying the standard for Gigabit Ethernet operation over optical
fiber. A companion 802.3ab task force expects to ratify the standard for
operation over Category 5 unshielded twisted pair (UTP) next March.
After that, the next major development will be the 802.3ad standard for
link aggregation. An IEEE task force was formed in July to develop and
approve the standard by March 2000.

The 802.3z standard defines two basic modes: the 1000Base-LX uses
long-wavelength laser transceivers to support links of up to 550 meters
with multi-mode fiber and 5 km with single-mode fiber; 1000Base-SX
employs short-wavelength laser transceivers to support links of 220 to
550 meters with multimode fiber, depending on its bandwidth. The task
force targeted 1000Base-SX at low-cost multimode fiber runs in
horizontal and shorter backbone applications. The 1000Base-LX is
intended for longer multimode building fiber backbones and single-mode
campus backbones.

For copper cabling, the 1000Base-CX standard supports short-haul
jumpers made from shielded twisted-pair for interconnecting equipment
clusters or for links within a switching closet or computer room of less
than 25 meters. The 1000Base-T standard for Category 5 UTP cable being
developed by the 802.3ab task force will utilize all four twisted pairs
to extend operation to a distance of 100 meters.

Given the high percentage of copper wiring in corporate networks,
development of the 1000Base-T standard is considered critical to Gigabit
Ethernet's adoption and growth. The draft standard calls for
five-level pulse amplitude modulation for transmission at 125 Mbaud over
each wire pair. Two data bits are encoded into a four-level signal, with
the fifth level for control. This results in 250 Mbps of data
transmitted over each of the four pairs.

The IEEE 802.3z task force is also working on a media-independent
interface that decouples the access protocol from the lower layer,
enabling separate development of additional physical layers based on
future advances in silicon technology and digital signal processing.

COMPATIBILITY CUES

Backward compatibility with the huge installed base of 10- and
100-Mbps Ethernet nodes was one of the objectives set by the IEEE 802.3
committee when it authorized the 802.3z task force to develop the 1-Gbps
specifications in July, 1996. Among the goals were use of the Ethernet
frame format and CSMA/CD (Carrier Sense Multiple Access/Collision
Detection) access method, and support for both half- and full-duplex
operation.

To make shared-media Gigabit Ethernet practical, however, the task
force had to modify the access method, which would otherwise have
limited implementations to spans of a few meters.

This limit is imposed by the CSMA/CD algorithm, which requires the
worst-case round-trip delay of the network to be less than or equal to
the transmission time of the shortest frame. With the 64-byte minimum
frame size used by 10- and 100-Mbps Ethernet, the network size for
1-Gbps operation would have to be less than 20 meters.

One solution, called carrier extension, increases the minimum frame
size to 512 bytes, lengthening the range to more than 150 meters but
causing inefficiencies when there is little data to send. Also, the
longer frame causes more collisions, reducing the efficiency even more.

To compensate, the IEEE 802.3z committee has adopted a further
enhancement, called packet bursting, that improves bandwidth utilization
for short frames and decreases the probability of collisions. With this
technique, a burst of short frames is appended to the channel-extended,
512-byte frame so that it avoids a collision. The overhead is shared
among several short frames.

Gigabit Ethernet supports new full-duplex operating modes for
switch-to-switch and switch-to-workstation connections. Full-duplex
operation requires no change in the minimum frame size since CSMA/CD is
suspended on such links to allow traffic to travel in both directions at
once. Its operation will be identical to that of Fast Ethernet.

Gigabit Ethernet has no provision for the quality of service (QoS)
needs of time-sensitive traffic, such as voice or video, but other
standards bodies are addressing these issues.

The IEEE 802.1p and 802.1q standards groups, for instance, provide
QoS over all forms of Ethernet by "tagging" packets with an
indication of their class of service so applications can communicate
each packet's priority to the internetworking devices. Also, the
Internet Engineering Task Force has issued a standard, called Resource
Reservation Protocol (RSVP), that lets end stations reserve bandwidth
for high-priority traffic.

Edwards is a datacommunications consultant who writes about network
computing technology and its business uses.

COPYRIGHT 1998 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.